1. Field of the Invention
The invention relates to the field of power electronics.
It is based on a power semiconductor component for high blocking voltages.
2. Discussion of Background
Such a power semiconductor component is described as MCT, for example in the article "Evolution of MOS Bipolar Power Semiconductor Technology" (B. J. Baliga in Proceedings of the IEEE, Vol. 76, No. 4, April 1988), as IGBT, for example in EP-A2-0,405,138, and as GTO, for example in U.S. Pat. No. 4,910,573.
The problems on which the application is based will be explained in greater detail in the text which follows, referring to an IGBT as it is described in the abovementioned European patent specification.
The IGBT described in this document exhibits a PT (punch-through) IGBT structure. It comprises a highly doped stop layer having a short carrier life. The n-type base consists of an epitaxially grown layer, the thickness of which is selected in accordance with the desired blocking voltage. This IGBT structure prevails today in the range of blocking voltages of up to 1 kV.
The abovementioned conventional IGBT structure is not suitable for high-voltage applications (e.g. 4.5 kV blocking voltage) for a number of reasons:
to the present day, there is no epitaxy technology available for guaranteeing the requirements for defect density and homogeneity of doping for very high blocking voltages; PA1 low turn-off losses can only be achieved with very short charge carrier lives; however, this entails an increase in the on-state losses which is unacceptable for high-voltage applications.
As a consequence, NPT (non-punch-through) IGBT structures are preferred today over the PT IGBTs for the range of higher blocking voltages (up to approximately 2 kV). Such an NPT IGBT structure is described in EP-A1-0,330,122. In each NPT power semiconductor component, the thickness of the semiconductor substrate is always greatly oversized for the required blocking voltage. This ensures that the field reins at a safe distance from the p+-type anode emitter even in the blocking case and thus no fatal short circuit can occur. A further reason for the great oversizing is based on keeping the magnitude of the tail currents and thus the magnitude of the turn-off losses at a low level. It is known that the tail currents will rise greatly if it is attempted to reduce the degree of oversizing of the substrate thickness. The increase in tail currents is attributed to a redistribution of the plasma in the quasi-neutral zone (from the anode end of the space charge zone up to the p+-type anode emitter).
The decay of the tail currents is essentially determined by the carrier life. Since the decay time constant is normally too long for achieving sufficiently low turn-off losses, means which support the decay time constant by charge carrier extraction are provided on the anode side. This can be done with the aid of a transparent emitter. In the case of GTOs it is also prior art to provide anode short circuits next to the p+-type anode emitter. The significant disadvantage of NPT power semiconductor structures for high-voltage applications consists in the uneconomic utilization of the substrate thickness. A typical NPT GTO for 4.5 kV exhibits a thickness of 700 .mu.m of the n-type base zone. A PT version for the same blocking voltage, in contrast, would only need about 400 .mu.m. The much lower on-state losses of a PT component can be used for achieving greatly reduced turn-off losses due to shortened carrier lives. As a consequence, a significant increase in the permissible switching frequency becomes possible.
The minimization of the substrate thickness is also of quite a significant importance for power diodes. In this manner, a PT structure can be used for reducing the recovered charge and thus the peak reverse recovery current to a minimum. However, it is known of such diode structures that an unwanted sharp discontinuity of the diode current occurs at the end of the depletion phase.
The practical embodiment of a PT power semiconductor component has previously also failed because of the connection of stop layer and anode and shorts. Because of the comparatively high conductivity of the stop layer, the greatest proportion of the electrons coming from the cathode flows into the anode short circuits. The p+-type anode emitters are then shorted too greatly and the basic advantage of the PT structure cannot be utilized because of the high on-state voltage. It has been attempted to prevent this by means of an extremely small proportion of anode shorts. However, the charge carrier extraction via the shorts is greatly impeded. As explained above, the level of the tail currents then increases with the consequence of unacceptably high turn-off losses.
The effects explained here occur both in the IGBT just discussed and in the MCT and GTO. In the diode, the chopping characteristic of the current during turn-off comes to the fore.